1. Field of the Invention
The present invention relates generally to a spread spectrum communication system and a method therefor. More particularly, the invention relates to a spread spectrum communication system and a method therefor of direct spread system.
2. Description of the Related Art
Conventionally, in a direct spread system, it has been taken a method for increasing and decreasing a transmission power depending upon increasing and decreasing of a reception bit error ratio for stabilizing communication quality. Namely, when the reception bit error ratio is increased, the transmission power is increased and when the reception bit error ratio is decreased, the transmission power is decreased.
In the conventional spread spectrum communication system, a band width of a channel is determined per system. When a data transmission rate is varied, the channel to be used is varied to other channel having a band width of double, four times and eight times wider band width. However, it should be questionable whether a channel band width of the conventional system is reasonable in view of communication quality and circuit capacity in practical use and currently in process of trial and error. Namely, even the band width unnecessary in a certain communication environment for a certain circuit at one timing, a broader band width may be required at other timing. Therefore, it is an effort for deriving an optimum value of the band width. On the other hand, due to abrupt depletion of a frequency resource in the recent years, a communication system of greater capacity has been desired. Furthermore, it has been demanded longer waiting period by further power saving, expansion of communication period, reduction of battery capacity and reduction of weight of the terminal.
Therefore, technologies for varying frequency band width depending upon conditions have been disclosed in Japanese Unexamined Patent Publication No. Heisei 6-14006 (hereinafter referred to as publication 1), Japanese Unexamined Patent Publication No. Heisei 6-46033 (hereinafter referred to as publication 2), Japanese Unexamined Patent Publication No. Heisei 6-216875 (hereinafter referred to as publication 3) and Japanese Unexamined Patent Publication No. Heisei 6-252881 (hereinafter referred to as publication 4).
The technology disclosed in the publication 1 is to widen the frequency band width when transmission quality is lowered. The technology disclosed in the publication 2 lowers error correction capacity when the bit error ratio is small to narrow frequency bandwidth by lowering information transmission speed. The technology disclosed in the publication 3 narrows the frequency band width by lowering process gain when the bit error ratio is small. The technology disclosed in the publication 4 is to make judgment of transmission path condition from the result of bit error ratio to narrow the bands using QPSK modulation when the transmission path condition is good, and to widen the band with BPSK modulation when the transmission path condition is no good.
The technologies of the publications 1 and 4 are common in widening the frequency band width when the transmission quality is lowered. On the other hand, in the technologies disclosed in the publications 2 and 3, there is no disclosure when the transmission quality is lowered.
However, the method for increasing and decreasing transmission power depending upon increasing and decreasing reception bit error ratio, will not provide any essential solution for selective fading, in which a certain band in the carrier wave band drops out. On the other hand, since increasing of transmission power is taken as measure for lowering of reception bit error ratio by the selective fading, it has been feared influence for multiplexed other channels.
On the other hand, in the technology to widen the frequency bandwidth when the transmission quality is lowered as disclosed in the publications 1 and 4, a defect is encountered in that widening of the frequency band width inherently cause lowering of the circuit use ratio per unit frequency.